iio: addac: add AD74115 driver

iio: addac: add AD74115 driver

The AD74115H is a single-channel, software-configurable, input and
output device for industrial control applications. The AD74115H
provides a wide range of use cases, integrated on a single chip.

These use cases include analog output, analog input, digital output,
digital input, resistance temperature detector (RTD), and thermocouple
measurement capability. The AD74115H also has an integrated HART modem.

A serial peripheral interface (SPI) is used to handle all communications
to the device, including communications with the HART modem. The digital
input and digital outputs can be accessed via the SPI or the
general-purpose input and output (GPIO) pins to support higher
speed data rates.

The device features a 16-bit, sigma-delta analog-to-digital converter
(ADC) and a 14-bit digital-to-analog converter (DAC).
The AD74115H contains a high accuracy 2.5 V on-chip reference that can
be used as the DAC and ADC reference.

Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20221117080916.411766-3-cosmin.tanislav@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>